Balances of this kind are primarily used to weigh small masses. Their preferred use is in the laboratory, and stringent requirements are imposed on measurement resolution. As the masses to be weighed are small, even minor extraneous factors can lead to inaccuracies. The maximum load that can be weighed is normally in the range from three to fifty grams, while the scale division is in most cases smaller than 10−7 times the maximum load.
The measurement results of these balances can be influenced even by small extraneous factors such as for example pressure fluctuations of the ambient air. These barometric fluctuations can have the consequence that the atmospheric pressure levels in the weighing compartment and in the weighing cell compartment differ from each other. An equalization of the pressure occurs by way of a stream of air flowing through the connecting passage between the weighing pan that is arranged in the weighing compartment and the weighing cell which is arranged in the weighing cell compartment. This air current can exert an upward or downward pull on the weighing pan which leads to inaccuracies of the weighing result. As a way to avoid these errors, some state-of-the-art balances have an additional opening between the weighing cell compartment and the weighing compartment in order to allow the atmospheric pressure to be equalized between the weighing compartment and the weighing cell compartment without causing an up- or downdraft on the weighing pan.
The temperature distribution inside the balance during operation can likewise have an influence on weighing accuracy. In particular the waste heat dissipated from the balance electronics has an influence on the temperature distribution. In many balances, the weighing cell, as well as the balance electronics, is accommodated in a weighing cell compartment or lower housing part which is arranged below the weighing compartment. As a consequence of the balance electronics being arranged below the weighing compartment, the problem occurs that the heat dissipated by the balance electronics can heat up the weighing cell compartment. This happens for example in such a way that the air in the lower part of the weighing compartment is being heated through a wall between the weighing cell compartment and the weighing compartment. Due to convection, the warmer air from the lower part of the weighing compartment flows upwards, which leads to air movements in the weighing compartment. The aim is to avoid these air movements, as they cause inaccuracies of the weighing result.
A possible solution whereby these air currents can be avoided is to arrange the balance electronics outside of the lower housing part.
In several commercially available balances, such an arrangement is realized with a design where the electronic system is relocated to a separate housing outside of the balance housing. By setting up the electronics in the separate housing, a thermal separation is established, and the waste heat dissipated by the balance electronics can no longer heat up the weighing compartment. However, the separate housing requires more space, which is a disadvantage especially in laboratories where the available space is limited. In other balances, the balance electronics are arranged in a lateral housing part of the balance, whereby the warming-up of the weighing cell compartment is reduced.
In the balance that is disclosed in U.S. Pat. No. 6,713,690, to Bierich, the relocation of the balance electronics from the lower housing part is realized by combining the indicator unit and the balance electronics in a separate housing part. The arrangement of a vertical plate between this separate housing part and the balance housing ensures that the radiated heat of the balance electronics is prevented as much as possible from having an influence on the weighing compartment. For the connecting cable between the separate housing part and the balance housing, a material with low thermal conductivity is chosen, whereby the heat transfer between the separate housing part and the balance housing is further reduced. A disadvantage of this setup is its complexity and the need for additional components such as for example the vertical plate.
It has been found to be advantageous if the air temperature in the upper part of the weighing compartment is higher than in the lower part because of the stable atmospheric stratification that is present in this case, which reduces the likelihood of air movements with their negative effects on the measurement result.
In German laid-open application 100 31 415 A1, to Oldendorf, different design configurations are presented which ensure this stable stratification of the air inside the weighing compartment. In one design concept, a heater is installed in the upper part of the weighing compartment to warm the air. In a further embodiment, the lower part and the upper part of the weighing compartment are connected with a vertical tube. Attached to this tube are means for generating an upward-directed heated air flow with a small cross-sectional profile. These design concepts have the disadvantage of using additional components. Furthermore, the air stream with its narrow profile causes a strong local disturbance of the air in the weighing compartment, which can lead to errors in the measurement result.
It is an object to provide a compact balance in which a stable stratification of the air in the weighing compartment during operation can be achieved in the simplest way possible.